The invention relates in particular to a yellow-emitting garnet phosphor for excitation by a light source with short wavelengths in the visible blue spectral region, with the result that white light is generated. A lamp (primarily a fluorescent lamp) or an LED (light-emitting diode) is particularly suitable as the light source.
WO 98/05078 has already disclosed a phosphor for light sources and an associated light source. In that document, the phosphor used is a garnet of the structure A3B5O12, the host lattice of which, as first component A, comprises at least one of the rare earths Y, Lu, Sc, La, Gd or Sm. Furthermore, one of the elements Al, Ga or In is used for the second component B. The only dopant used is Ce.
A very similar phosphor is known from WO 97/50132. The dopant used in that document is either Ce or Tb. While Ce emits in the yellow spectral region, the emission from Tb is in the green spectral region. In both cases, the complementary color principle (blue-emitting light source and yellow-emitting phosphor) is used to achieve a white luminous color.
Finally, EP-A 124 175 describes a fluorescent lamp which, in addition to a mercury fill, contains a plurality of phosphors. These are excited by UV radiation (254 nm) or also by short-wave radiation at 460 nm. Three phosphors are selected in such a way that they add up to form white (color mixture).
According to the invention, for light sources from which the emission lies in the short-Wave optical spectral region, a phosphor which has a garnet structure A3B5O12 and which is doped with Ce is used, the second component B representing at least one of the elements Al and Ga and the first component A containing terbium. Surprisingly, it has been found that under particular circumstances, namely under blue excitation in the range from 420 to 490 nm, terbium (Tb) is suitable as a constituent of the host lattice (first component of the garnet) for a yellow-emitting phosphor, the activator of which is cerium. Previously, in this context Tb has only been considered as an activator or coactivator, together with cerium, for emission in the green region, if excitation is produced by cathode rays (electrons) or short-wave UV photons (GB-A 1 600 492 and EP-A 208 713).
In this case, terbium, as the principal constituent of the first component A of the garnet, can be used on its own or together with at least one of the rare earths Y, Gd, La and/or Lu.
At least one of the elements Al or Ga is used as the second component. The second component B may additionally contain In. The activator is cerium. In a particularly preferred embodiment, a garnet of the structure
(Tb1xe2x88x92xxe2x88x92yRExCey)3(Al,Ga)5O12, where
RE=Y, Gd, La and/or Lu;
0xe2x89xa6xxe2x89xa60.5xe2x88x92y;
0 less than y less than 0.1 is used.
The phosphor absorbs in the range from 420 to 490 nm and can thus be excited by the radiation from a blue light source, which is in particular the radiation source for a lamp or LED. Good results have been achieved with a blue LED whose emission peak was at 430 to 470 nm. The emission peak of the Tb-garnet: Ce phosphor is at approximately 550 nm.
This phosphor is particularly useful for use in a white LED based on the combination of a blue LED with the Tb-garnet-containing phosphor, which is excited by absorption of part of the emission from the blue LED and the emission from which supplements a remaining radiation from the LED, to form white light.
A Ga(In)N-LED is particularly suitable as the blue LED, but any other route for producing a blue LED which emits in the range from 420 to 490 nm is also suitable. 430 to 470 nm is particularly recommended as the principal emission region, since this is where efficiency is highest.
By selecting the type and quantity of rare earths, it is possible to fine-tune the location of the absorption and emission bands, in a similar way to that which is known from the literature for other phosphors of type YAG:Ce. In conjunction with light-emitting diodes, it is particularly suitable for x to be 0.25xe2x89xa6xxe2x89xa60.5xe2x88x92y.
The particularly preferred range for y is 0.02 less than y less than 0.06.
The phosphor according to the invention is also suitable for combination with other phosphors.
A garnet of structure
(TbxRElxe2x88x92xxe2x88x92yCey)3(Al,Ga)5O12,
where RE=Y, Gd, La and/or Lu;
0xe2x89xa6xxe2x89xa60.02, in particular x=0.01;
0 less than y less than 0.1 has proven particularly suitable as the phosphor. Y frequently lies in the range from 0.01 to 0.05.
Generally, relatively small amounts of Tb in the host lattice serve primarily to improve the properties of known cerium-activated phosphors, while the addition of relatively large amounts of Tb can be used in a controlled way in particular to shift the wavelength of the emission from known cerium-activated phosphors. Therefore, a high proportion of Tb is particularly suitable for white LEDs with a low color temperature of below 5000 K.